1. Field of the Invention
This invention relates to metered-dose dispensing valves and in particular to valves for dispensing medicament from pressurized aerosol containers.
2. Description of the Related Art
Metering valves for use with pressurized metered dose inhalers (MDIs) need to meet certain performance criteria with regard to the sampling of the medicinal formulation, which is generally in the form of suspension of micronized drug particles in an aerosol propellant system. Specifically, the valve must sample the formulation homogeneously in order that the correct dose of medicament is delivered to the patient on each actuation of the valve. Most commercially available valves fulfill this requirement when the formulation is well dispersed.
It has been found that valve types vary considerably in their ability to dose high potency low concentration formulations. This problem is particularly significant when the formulations are not well dispersed as may occur in various conditions in the field, for instance when a formulation has been allowed to stand for a while such that creaming has taken place and the valve is actuated without shaking the product.
In such circumstances, depending on the valve design, it may be possible to deliver either a dose within intended specification or a dose of up to about twenty times the target value. This problem is of concern since it cannot be assumed that a patient will always shake an MDI product before use.
In addition, a second effect has been identified where valve design may influence the consistency of drug delivery. This effect concerns a tendency for drug particles to migrate to the metering tank of the valve during storage or transit, with a consequent elevation in the amount of drug delivered in the first shot. It has been found that elevated drug doses may be obtained due to this effect, particularly with high potency drugs for which there is a very small total quantity of active substance in the aerosol container.
A third effect has been identified which may cause erratic dosing and this concerns the ability of drug to cream out of the metering tank of valves stored in the stem down position when the valve is of a type having an axial point of product entry.